Editing Canopus (section)

==Observation==
[[File:Carina constellation map.svg|thumb|left|The constellation [[Carina (constellation)|Carina]] with Canopus towards the right (west)]]
The Muslim astronomer [[Ibn Rushd]] went to [[Marrakesh]] (in Morocco) to observe the star in 1153, as it was invisible in his native [[Córdoba, Spain|Córdoba]], [[Al-Andalus]]. He used the different visibility in different latitudes to argue that the [[Spherical Earth|Earth is round]], following Aristotle's argument which held that such an observation was only possible if the Earth was a relatively small sphere.<ref name=ibn_rushd>{{cite encyclopedia|first1=Juan|last1=Vernet|author-link1=Juan Vernet|first2=Julio|last2=Samsó|author-link2=Julio Samsó |title=The development of Arabic science in Andalusia |page=264|editor=[[Roshdi Rashed]]|encyclopedia=Encyclopedia of the History of Arabic Science | date=1996|publisher=Routledge|isbn=978-0-415-12410-2|oclc=912501823}}</ref>

English explorer [[Robert Hues]] brought Canopus to the attention of European observers in his 1592 work ''Tractatus de Globis'', along with [[Achernar]] and [[Alpha Centauri]], noting:<blockquote>"Now, therefore, there are but three [[First-magnitude star|Stars of the first magnitude]] that I could perceive in all those parts which are never seene here in England. The first of these is that bright Star in the sterne of [[Argo Navis|Argo]] which they call Canobus. The second is in the end of [[Eridanus (constellation)|Eridanus]]. The third is in the right foote of the [[Centaurus|Centaure]]."<ref>Knobel, p. 416.</ref></blockquote>

[[File:Vela and Surrounding Constellations (ground-based image).jpg|right|thumb|Wide angle view showing Canopus and other prominent stars with the Milky Way|alt=A field of stars against the Milky Way background with the prominent stars and constellations labelled]]
In the [[Southern Hemisphere]], Canopus and [[Sirius]] are both visible high in the sky simultaneously, and reach a [[meridian (astronomy)|meridian]] just {{val|21|u=minutes}} apart. Brighter than [[first-magnitude star|first magnitude]], Canopus can be seen by naked eye in the early twilight. Mostly visible in mid to late summer in the Southern Hemisphere, Canopus [[Culmination|culminates]] at midnight on December 27,<ref name="motz" /> and at 9 PM on February 11.<ref name="Schaaf257">Schaaf, p. 257.</ref>

When seen from latitudes south of {{DEC|37|18}}&nbsp;S, Canopus is a [[circumpolar star]]. Since Canopus is so far south in the sky, it never rises in mid- to far-northern latitudes; in theory the northern limit of visibility is latitude {{DEC|37|18}} north. This is just south of [[Athens]], [[San Francisco]], and [[Seoul]], and very close to [[Seville]] and [[Agrigento]]. It is almost exactly the latitude of [[Lick Observatory]] on [[Mount Hamilton (California)|Mt. Hamilton, California]], from which it is readily visible because of the effects of elevation and [[atmospheric refraction]], which add another degree to its apparent altitude. Under ideal conditions, it can be spotted as far north as latitude {{DEC|37|31}} from the Pacific coast.<ref>D. Gieringer, "Exploring the Tropic of Canopus", ''Astronomy'', December 1985, p.24.</ref> Another northernmost record of visibility came from [[Mount Nemrut]] in Turkey, latitude {{DEC|37|59}}.<ref>{{cite web |last1=Tezel |first1=Tunç |author-link=Tunç Tezel |url=http://www.twanight.org/newTWAN/photos.asp?ID=3004572 |title=Zodiacal Light and Nemrut Heritage |work=The World At Night (TWAN) |access-date=17 March 2014 |date=8 Oct 2013 |archive-url=https://web.archive.org/web/20140317134955/http://www.twanight.org/newTWAN/photos.asp?ID=3004572 |archive-date=17 March 2014 |url-status=dead }}</ref> It is more easily visible in places such as the Gulf Coast and Florida, and the island of [[Crete]] (Greece) where the best season for viewing it around 9 p.m. is during late January and early February.<ref name="motz">{{cite book|last1=Motz|first1=Lloyd|last2=Nathanson|first2=Carol|title=The Constellations: An Enthusiast's Guide to the Night Sky|publisher=Aurum Press|location=London, United Kingdom|date=1991|pages=376–77|isbn=1-85410-088-2}}</ref>

Canopus has a B–V [[color index]] of +0.15—where 0 is a blue-white—indicating it is essentially white, although it has been described as yellow-white. Canopus' spectral type has been given as F0 and the incrementally warmer A9. It is less yellow than [[Altair]] or [[Procyon]], with indices measured as 0.22 and 0.42, respectively.<ref name=Hoffleit1991>{{cite book | title=Bright Star Catalogue | edition=5th Revised | last1=Hoffleit | first1=D. | last2=Warren Jr. | first2=W. H. | date=1991 | publisher=[[Centre de Données astronomiques de Strasbourg|CDS]] }}</ref> Some observers may have perceived Canopus as yellow-tinged because it is low in the sky and hence subject to atmospheric effects.<ref name="Schaaf112">Schaaf, pp. 112–13.</ref> [[Patrick Moore]] said that it never appeared anything but white to him.<ref name="moore2000">{{cite book | first1=Patrick | last1=Moore | title=Exploring the night sky with binoculars | edition=4th | publisher=Cambridge University Press | year=2000 | isbn=9780521793902 | page=[https://archive.org/details/exploringnightsk00moor/page/69 69] | url=https://archive.org/details/exploringnightsk00moor| url-access=registration }}</ref> The [[bolometric correction]] for Canopus is 0.00,<ref name=smiljanic2006/> indicating that the visual [[absolute magnitude]] and bolometric absolute magnitude are equal.

Canopus was previously proposed to be a member of the [[Scorpius–Centaurus association]], however it is not located near the subgroups of that association, and has not been included as a Sco-Cen member in kinematic studies that used [[Hipparcos]] astrometric data.<ref name="deZeeuw">{{cite journal|last1=de Zeeuw|first1=P.T. |last2=Hoogerwerf|first2=R. |last3=de Bruijne|first3=J.H.J |last4=Brown|first4=A.G.A |last5=Blaauw|first5=A. |date=1999|title=A HIPPARCOS Census of the Nearby OB Associations |journal=The Astronomical Journal |volume=117 |issue=1 |pages=354–399|bibcode=1999AJ....117..354D |doi=10.1086/300682|arxiv = astro-ph/9809227 |s2cid=16098861 }}</ref> Canopus is not thought to be a member of any nearby young stellar groups.<ref name=mamajek>{{cite AV media | url=https://figshare.com/articles/Canopus_B_A_Candidate_Common_Proper_Motion_Companion_to_the_Second_Brightest_Star/1132696 | title=Canopus B: A Candidate Common Proper Motion Companion to the Second Brightest Star | last=Mamajek | first=Eric | year=2014 | publisher=Figshare | doi=10.6084/m9.figshare.1132696.v3 | access-date=2020-09-10 }}</ref>  In 2014, astronomer Eric Mamajek reported that an extremely magnetically active M dwarf (having strong coronal X-ray emission), 1.16 degrees south of Canopus, appears to share a [[common proper motion]] with Canopus. The projected separation of the M dwarf 2MASS J06234738-5351131 ("Canopus B") is approximately 1.9 parsecs. However, despite this large separation, it is still within the estimated tidal radius (2.9 parsecs) for the massive star Canopus.<ref name="mamajek"/>

Since it is more luminous than any star closer to Earth, Canopus has been the brightest star in the night sky during three epochs over the past four million years. Other stars appear brighter only during relatively temporary periods, during which they are passing the Solar System much closer than Canopus. About 90,000 years ago, [[Sirius]] moved close enough that it became brighter than Canopus, and that will remain so for another 210,000 years. But in 480,000 years, as Sirius moves further away and appears fainter, Canopus will once again be the brightest, and will remain so for a period of about 510,000 years.<ref name="tomkin98">{{cite journal|last=Tomkin|first=Jocelyn|date=April 1998|title=Once and Future Celestial Kings|journal=Sky and Telescope|volume=95|issue=4|pages=59–63|bibcode=1998S&T....95d..59T}}</ref><!-- this all comes from same paper -->

===Role in navigation===
[[File:Constellation_Carina.jpg|thumb|right|upright|Canopus is the brightest star in the constellation of Carina (top).]]
The southeastern wall of the [[Kaaba]] in [[Mecca]] is aligned with the rising point of Canopus, and is also named ''Janūb''.<ref>{{cite book|author=George Nicholas Atiyeh|title=The Book in the Islamic World: The Written Word and Communication in the Middle East|url=https://books.google.com/books?id=t4LEfpCW_kQC|date=1 January 1995|publisher=SUNY Press|isbn=978-0-7914-2473-5}}</ref> The [[Negev Bedouin|Bedouin]] people of the [[Negev]] and [[Sinai Peninsula|Sinai]] knew Canopus as ''Suhayl'', and used it and [[Polaris]] as the two principal stars for navigation at night. Because it disappears below the horizon in those regions, it became associated with a changeable nature, as opposed to always-visible Polaris, which was circumpolar and hence 'steadfast'.<ref>{{cite journal |last=Bailey |first=Clinton |date=1974 |title=Bedouin Star-Lore in Sinai and the Negev |journal=Bulletin of the School of Oriental and African Studies, University of London |volume=37 |issue=3 |pages=580–96 |type=abstract |doi=10.1017/S0041977X00127491 |jstor=613801|s2cid=131527348 }}</ref>

The south [[celestial pole]] can be approximately located using Canopus and two different bright stars. The first, [[Achernar]], makes an [[equilateral triangle]] between the stars and the south pole. One can also locate the pole more roughly using an imaginary line between Sirius and Canopus; Canopus will be approximately at the midpoint, being {{DEC|36}} one way to Sirius and {{DEC|37}} to the pole.<ref>{{Cite book| last1 = Heifetz| first1 = Milton| last2 = Tirion| first2 = Wil| title = A Walk Through the Heavens: A Guide to Stars and Constellations and Their Legends | date = 2007| publisher = [[Cambridge University Press]]| location = Cambridge | page = 38 | isbn=978-1-139-46138-2}}</ref>

Canopus's brightness and location well off the [[ecliptic]] make it useful for space navigation. Many spacecraft carry a special camera known as a "Canopus [[star tracker|Star Tracker]]" plus a Sun sensor for [[Orientation (geometry)|attitude]] determination. [[Mariner 4]] used Canopus for [[Spin-stabilisation|second axis stabilisation]] (after locking on the Sun) in 1964, the first time a star had been used.<ref>{{cite book |last1=United States. National Aeronautics and Space Administration. Scientific and Technical Information Division |title=Astronautics and Aeronautics, 1964: Chronology on Science, Technology and Policy |date=1965 |publisher=Scientific and Technical Information Division, National Aeronautics and Space Administration |page=398 |url=https://books.google.com/books?id=ckkCAAAAIAAJ&pg=PA398}}</ref>